Try to avoid multi-threaded operations with Xapian.
This remove the thread workers during indexing, and avoids the indexing
background thread. So, mu4e has to wait once again during indexing.
We can improve upon that, but first we need to know if it avoids the
problem of issue #2756.
Add store::consume_message, which is like add message but std::move from
the caller such that the messages longer has copies (with
Xapian::Document) on the caller side; this is to avoid threading issues.
Instead of handling transactions in the store, handle it in xapian-db.
Make the code a bit more natural / cleaner-out
Handle transaction automatically (with a batch-size) and add some RAII
Transaction object, which makes all database interaction transactable
for the duration. So, no more need for explicit parameters to
add_message while indexing.
